A simple radioimmunoassay has been developed utilizing guanylate cyclase isolated from sea urchin sperms. The enzyme was purified as reported last year to single electrophoretic band purity and was iodinated with (125I) Bolton-Hunter reagent. The antigen antibody complex was precipitated with anti IgG. The assay was capable of measuring subpicomoles of guanylate cyclase or antibody. This method has been used to detect antiguanylate cyclase antibody production in myeloma hybridoma cell cultures.